Current measurement system

ABSTRACT

A measurement system for measuring an input electrical current (Ics) from a current source (CS) and generating a current measurement signal, comprising a current measuring circuit ( 70 ) having a first input terminal ( 72 ) connected to the current source and an output terminal ( 74 ) for providing the current measurement signal. The current measuring circuit further comprises one or more power supply terminals ( 75, 76 ) arranged to receive one or more voltages from a power supply ( 77   a,    77   b ) for powering the current measuring circuit. The current measuring circuit also comprises a first voltage source (VD) coupled to the one or more power supply terminals, the first voltage source providing a disturbance voltage to the one or more power supply terminals, the disturbance voltage representing a voltage at the first input terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a capacitive sensor for measuringdistance, in particular capacitive sensor for measuring distance to atarget in a lithography apparatus.

2. Description of the Related Art

In many applications it is important to measure an electrical currentvery precisely. For example, charged particle and optical lithographymachines and inspection machines for example, typically require highlyaccurate measurement of the distance from the final lens element of themachine to the surface of a wafer or other target to be exposed orinspected. These machines and others with movable parts often requireprecise alignment of various parts which may be achieved by measuringdistance from the moveable part to a reference point. Capacitive sensorsmay be used in such applications requiring fine position or distancemeasurement. When a capacitive sensor is energized, an electricalcurrent flows through the sensor which varies in dependence on thedistance between the sensor element and an opposing surface. A precisemeasurement of this current may be used to accurately determine themeasured distance.

To measure an electric current, a measurement circuit may be used havingthe current to be measured as the input and providing a measurementsignal as an output, often in the form of a voltage which may be furtherprocessed and converted to a digital signal. There are several factorswhich contribute to errors in such measurement circuits. These includestray impedance in the input circuitry of the measuring circuit, alimited common mode rejection ratio (CMRR) of the input circuitry, andinaccuracy on the transfer function of the measurement circuitindependent of common mode. The value of such stray impedance maychange, e.g. depending on factors such as temperature, and disturbanceson the input may also change with time. This makes it difficult tocompensate for these effects.

It is often necessary to locate the electronic measurement circuits usedfor driving the capacitive sensors and for generating the desiredmeasurement signals at a distance from the sensors due to theinhospitable environment in which the sensors are located or lack of asuitable place to locate the circuits close to the sensors. In modernlithography applications such as EUV and charged particle systems, thesensors are typically located in a vacuum environment that is verysensitive to contaminants and outside disturbances, and which createsproblems with heat removal from electronic circuits if they are locatedin the vacuum environment, and impedes access for maintenance for suchcircuits.

The wiring connections between the sensors and remotely located drivingand measurement circuits introduce parasitic capacitances into thesystem which affect the reading of the sensor. If the measuring circuitscould be located at the sensor probe, the sensor current could bemeasured directly and accurately. Because of these parallel parasiticcapacitances introduced by the cabling system, measurement of currentflow in the sensor is often avoided in systems with remotely locatedmeasuring circuits. Conventional solutions introduce measurement errorswhich need to be taken into account, usually by calibrating the combinedsensor and wiring installation. The longer the wiring connection, themore severe these problems become.

The requirement to calibrate the sensors in combination with the sensorwiring reduces flexibility in designing and building sensor systems andincreases their cost, and it adds a requirement for recalibrationwhenever a sensor or its wiring is replaced, making such a replacementcomplex, time-consuming, and expensive.

BRIEF SUMMARY OF THE INVENTION

The invention seeks to solve or reduce the above drawbacks to provide animproved measurement system for measuring an input electrical currentfrom a current source and generating a current measurement signal,comprising a current measuring circuit having a first input terminalconnected to the current source and an output terminal for providing thecurrent measurement signal. The current measuring circuit furthercomprises one or more power supply terminals arranged to receive one ormore voltages from a power supply for powering the current measuringcircuit. The current measuring circuit also comprises a first voltagesource coupled to the one or more power supply terminals, the firstvoltage source providing a disturbance voltage to the one or more powersupply terminals, the disturbance voltage representing a voltage at thefirst input terminal.

The measurement system may further comprise a difference circuitarranged to subtract a voltage generated by the first voltage sourcefrom a signal at the output terminal of the current measuring circuit togenerate the current measurement signal.

The first voltage source may be connected to the first input terminal ofthe current measuring circuit for driving a load to form the currentsource. The load may comprise a capacitive sensor for generating acurrent which varies in dependence on distance between the capacitivesensor and a target. The load may be connected to the first inputterminal of the current measuring circuit by a cable comprising a sensorwire and a shield conductor, wherein the sensor wire is connected inseries between the load and the first input terminal and the shieldconductor is connected to the first voltage source.

An output terminal of the first voltage source may be coupled via one ormore capacitors to the one or more power supply terminals of the currentmeasuring circuit. The current measuring circuit may comprise acurrent-to-voltage converter.

The current measuring circuit may comprise an operational amplifier, anegative input terminal of the operational amplifier serving as thefirst input terminal of the current measuring circuit and an outputterminal of the operational amplifier serving as the output terminal ofthe current measuring circuit, the operational amplifier furthercomprising a positive input terminal and one or more power supplyterminals, wherein the positive input terminal of the operationalamplifier is electrically connected to the one or more power supplyterminals of the operational amplifier. The positive input terminal ofthe operational amplifier may be electrically connected to the one ormore power supply terminals of the operational amplifier via one or morecapacitors.

The first voltage source may be used to generate a voltage with atriangular waveform, and the current source may generate a current witha substantially square waveform.

In another aspect, the invention relates to a method for measuring aninput electrical current from a current source and generating a currentmeasurement signal. The method comprises the steps of providing theinput current to a first input terminal of a current measuring circuit,the measuring circuit having one or more power supply terminals arrangedto receive one or more voltages from a power supply for powering thecurrent measuring circuit, providing a disturbance voltage to the one ormore power supply terminals, the disturbance voltage representing avoltage at the first input terminal, and generating an output signal atan output terminal of the current measuring circuit representing theinput electrical current at the first input terminal of the currentmeasuring circuit.

The method may further comprise subtracting the disturbance voltage fromthe output signal at the output terminal of the current measuringcircuit to generate the current measurement signal. The method may alsocomprise driving a load with a voltage to generate the input electricalcurrent at the first input terminal of the current measuring circuit,and the load may comprise a capacitive sensor for generating a currentwhich varies in dependence on distance between the capacitive sensor anda target.

The method may further comprise connecting the load to the first inputterminal of the current measuring circuit by a cable comprising a sensorwire and a shield conductor, wherein the sensor wire is connected inseries between the load and the first input terminal and the shieldconductor is energized with substantially the same voltage used to drivethe load.

The disturbance voltage may be provided to the one or more power supplyterminals via one or more capacitors, and may be isolated from the powersupply voltages by one or more inductors.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the invention will be further explained withreference to embodiments shown in the drawings wherein:

FIG. 1 is a diagram of a capacitive sensor probe and grounded conductivetarget;

FIG. 2 is a diagram of two capacitive sensor probes in a differentialmeasurement arrangement with a non-grounded target;

FIG. 3 is a diagram of an active guarding circuit and a coaxial cable incombination with a voltage source and current measurement circuit;

FIG. 4 is a diagram of a measurement circuit with a voltage source andshield driver driving both a sensor wire and shield conductor in adifferential sensor pair arrangement;

FIG. 5 is a diagram of a variation of the measurement circuit of FIG. 4with shield drivers integrated into the voltage sources;

FIG. 6 is a diagram of a triaxial cable used for the measurement circuitof FIG. 4 or FIG. 5;

FIG. 7A is a diagram of triangular AC voltage waveform for driving acapacitive sensor;

FIG. 7B is a diagram of an ideal AC current waveform resulting from thetriangular voltage waveform of FIG. 7A;

FIG. 7C is a diagram of a AC current waveform resulting in practice fromthe triangular voltage waveform of FIG. 7A;

FIG. 8 is a cross sectional view of a thin film capacitive sensorconnected to a triaxial sensor cable;

FIG. 9A is a cross sectional view of a thin film capacitive sensor pair;

FIG. 9B is a top view of the thin film capacitive sensor pair of FIG.9A;

FIG. 10 is a simplified diagram of capacitive sensors and measurementcircuit implemented for distance measurement in a charged particlelithography machine;

FIG. 11 is a simplified view of a modular lithography system withmultiple sets of thin film capacitive sensors for position measurementof movable parts;

FIG. 12 is a simplified functional diagram of a current measuringcircuit with an input voltage disturbance fed into the power supply;

FIG. 13 is a simplified diagram of the current measuring circuit of FIG.12 with a load driven by a voltage source;

FIG. 14 is a simplified diagram of a current measuring circuitimplemented with an operational amplifier;

FIG. 15 is a simplified diagram of the current measuring circuit of FIG.14 for measuring current from a capacitive sensor;

FIGS. 16A-C are waveform diagrams of signals for the current measuringcircuit of FIGS. 13-15;

FIG. 17 is a simplified circuit diagram of one embodiment of a currentmeasurement system;

FIGS. 18A-K are examples of waveforms generated in the circuit of FIG.17; and

FIG. 19 is a simplified block diagram of layout of electronic circuitsfor a current measurement system.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following is a description of various embodiments of the invention,given by way of example only and with reference to the drawings.

A capacitive sensor uses a homogeneous electric field set up between twoconductive surfaces. Over short distances, the applied voltage isproportional to the distance between the surfaces. Single-plate sensorsmeasure the distance between a single sensor plate and an electricallyconductive target surface.

FIG. 1 shows a single capacitive sensor probe 1 measuring the positionof or separation distance to grounded conductive target 2. When suppliedwith an AC current, current will flow along a path 3 from the sensor tothe target through the sensor-target capacitance 4, and from the targetto ground through the target-ground impedance 5. The voltage across thesensor will vary in dependence on the distance separating the sensorprobe and the surface of the target, and measuring this voltage willprovide a measurement of target position or distance from the sensorprobe to the target. The accuracy of the measurement depends on howaccurately the sensor can measure the sensor-target capacitance 4.

FIG. 2 shows an arrangement of two capacitive sensor probes 1 a and 1 bfor a differential measurement of the position or separation distance totarget 2. The sensors are supplied with an AC current with a phaseoffset of 180 degrees, so that current will flow along a path 6 from onesensor to the target through the sensor-target capacitance 4 a, and fromthe target to the other sensor through the other sensor-targetcapacitance 4 b. This arrangement for driving the two sensors without-of-phase signals is effective to avoid flow of current through thetarget to ground, and minimizes the effect of the target to groundimpedance. It is also useful for an ungrounded target as it allowscurrent to flow from one sensor to the other without needing a groundedreturn path.

The capacitive sensor may be energized by an AC voltage source or ACcurrent source, and the resulting voltage across the sensor or currentthrough the sensor is measured. The measurement signal generated isdependent on the sensor-to-target capacitance of the sensor. The systemcan be calibrated to the measurement capacitor and to measure thecurrent/voltage.

The environment in which capacitive sensors are typically applied inindustrial applications is often an unsuitable location for the currentor voltage source for driving the capacitive sensors and the measurementcircuits for processing signals from the sensors. As a result, thedriving source and measurement circuits are typically located remotelyfrom the sensors, requiring a cable connection to the sensor. A cablingconnection between the sensors and the remote circuits will introduceadditional undesirable capacitances in the system, even when the cableis short.

FIG. 3 is a diagram showing such a cable connection and the capacitancesintroduced by the cable into the sensor system. The sensor-to-targetcapacitance 4 is the capacitance to be measured (also referred to as thesensor capacitance), which depends on the distance between the sensorand the target. The cable 30 comprises a central conductor 31 andcoaxial shield conductor 32, and the cable introduces stray capacitance36, referred to as the cable capacitance, between the sensor wire 31 andshield 32, and stray ground capacitance 37 between the shield 32 andground.

A voltage source 20 is connected through a current measurement circuit21 to one end of the sensor wire 31 and the measurement electrode of thecapacitive sensor is connected to the other end of the sensor wire. Thevoltage source 20 supplies an AC voltage to energize the capacitivesensor 1, and the current measurement circuit 21 measures the currentflowing in the sensor wire 31 through the capacitive sensor 1. Thecurrent flowing through the sensor wire 31 varies in dependence on thesensor capacitance 4, which varies in dependence on the distance beingmeasured by the sensor.

The current flowing in the sensor wire 31 will include a component dueto current flowing through the sensor capacitance 4 and also a componentdue to current flowing through the cable capacitance 36. The cablecapacitance 36 should be small in comparison to the sensor capacitance 4because large stray capacitances increase the proportion of currentflowing through the stray capacitances in comparison to the currentflowing through the sensor capacitance desired to be measured, andreduces the sensitivity of the measurement. However, the cablecapacitance is typically large and has an adverse effect on sensorsystem sensitivity.

Active guarding may be used to minimize the effect of the cablecapacitance. FIG. 3 shows a conventional arrangement with a shielddriver 24 comprising a unitary gain amplifier/buffer with an inputconnected to the end of sensor wire 31 and an output connected to shield32. The shield driver 24 energizes shield 32 with (essentially) the samevoltage as present on sensor wire 31. Since the sensor wire 31 andshield conductor 32 have almost the same voltage on them, there is onlya small voltage difference between them and only a small current flowthrough the cable capacitance 36, and the effect of cable capacitance 36between the conductors is reduced. In practice, the gain of the shielddriver only approaches a gain of 1.0 and some deviation in the gain mustbe expected. Any such deviation in gain results in a small voltagedifference between the shield 32 and sensor wire 31, so that there is avoltage across the cable capacitance 36. This causes a current flowthrough the capacitance 36 and reduces the sensitivity of the sensorsystem. For long cables (with higher cable capacitances) and highermeasurement frequencies this arrangement becomes even less effective.

Current flow through stray cable-to-ground capacitance 37 is supplied byshield driver 24. The input current to shield driver 24 will contributeto the current measured by current measurement circuit 21 resulting inerror, but the shield driver has a high input impedance and its inputcurrent is relatively small so that the resulting error is small.However, for long cables and higher measurement frequencies thisarrangement is difficult to realize. The shield driver also has someinput capacitance, which will draw additional current. The measuredcapacitance is the sum of the sensor capacitance 4 and these additionalerror capacitances; the deviation from unit gain of the shield driver 24multiplied by the stray capacitance 36, and the input capacitance of theshield driver 24.

The measurement error can be reduced by rearranging the circuit as shownin FIG. 4. This arrangement is for driving two capacitive sensors in adifferential pair arrangement. For position measurement systems in whichthe target (or part of the target) is not a conductor or is otherwiseisolated from ground, a second sensor and second measurement circuitwith an inverted driver may be used in a differential pair arrangementsuch as shown in FIG. 4.

The output of voltage source 20 a is connected to the input of shielddriver 24 a, the output of shield driver 24 a is connected to oneterminal of current measurement circuit 21 a, and the other terminal ofmeasurement circuit 21 a is connected to sensor wire 31 a. The samearrangement is used for voltage source 20 b, shield driver 24 b, currentmeasurement circuit 21 b, and sensor wire 31 b. The Voltage sources 20 aand 20 b generate AC voltage waveforms phase offset by 180 degrees toeach other. The target conducts the alternating current between the twosensors 1 a and 1 b through the two sensor capacitances 4 a and 4 b. Thetarget behaves like a virtual ground for the two measurement systems;this is optimal if the sensor capacitances 4 a and 4 b are equal. Thepotential of the target will be removed as a common mode disturbancewhen the difference between the two current measurements 22 a and 22 bis calculated.

Moving the input of the shield driver to a point ‘before’ the currentmeasurement omits the input capacitance of the shield driver from thecapacitance measurement, thus eliminating this component of error fromthe measurement. This can also be viewed as a feed forward of the shielddriver output to the shield conductor. The voltage source output isstill transferred to the sensor wire, and is also directly connected todrive the shield conductor, instead of buffering the sensor wire voltagein order to load the shield conductor. Connecting the shield driver inseries between the voltage source and measurement circuit has theadditional benefit of removing error caused by deviation from unity gainof shield driver, because the shield driver output is connected to boththe sensor wire (through the measurement circuit) and the shieldconductor.

FIG. 5 shows a further refinement, with the same configuration as inFIG. 4 but with the separate shield driver 24 a/24 b omitted, thisfunction being integrated with the voltage source 20 a/20 b to drive allcapacitances in the system. This arrangement uses the same driver forboth the sensor wire and shield conductor, and measures the currentflowing in the sensor wire. The resulting system achieves simplicitywhile eliminating sources of measurement error present in conventionalarrangements.

The arrangement of FIGS. 4 and 5 effectively eliminates any voltagedifference between the sensor wire 31 and shield conductor 32 so thatthere is negligible voltage across the cable capacitance 36. Thiseffectively eliminates current through the cable capacitance 36 and thecurrent measured by circuit 21 is effectively only the current throughthe sensor capacitance 1. The input impedance of the current measurementcircuit is made sufficiently low that the voltages supplied to thesensor wire and shield conductor are nearly equal.

Current through the capacitance 37 between the shield 32 and ground issupplied from the voltage source 20 or separate shield driver 24, andthis current does not form part of the measured current and has only asecond order effect on the voltage at the output of the voltage source.Any deviation from unity gain of the shield driver and the effect ofinput capacitance of the shield driver are both eliminated in thisarrangement.

In effect the arrangement in FIGS. 4 and 5 results in driving the shieldconductor 32 and coupling the shield to the sensor wire 31 so that thevoltage on the sensor wire follows the voltage on the shield. This isthe reverse of the conventional arrangement in which the sensor wire isdriven and the voltage on the sensor wire is copied onto the shieldconductor. In this design the focus is changed from being primarilydirected to measuring the current through the sensor (and therebymeasuring the sensor capacitance and distance value) while accountingfor the current leakage due to the parasitic capacitances, to primarilyfocusing on providing a suitable environment for an accurate sensorcurrent measurement by steering the shield conductor voltage, realizingthis to be the main problem and measuring the sensor current to be alesser problem.

A grounded outer shield conductor may also be added to theconfigurations of FIGS. 4 and 5 to reduce interference from any nearbysources of noise. FIG. 6 shows a cable 30 a with a grounded outer shieldconductor 33 a placed around the (inner) shield conductor 32 a. Thecable in this embodiment is a triaxial cable, the grounded shield 33 aforming the outermost conductor. The grounded shield is preferablyconnected to a separate ground at the remote end of the cable, e.g. nearthe measurement circuit 21 a. This ground is a shielding ground and ispreferably not connected to any ground at the sensor. Interference withother nearby systems can be reduced using a grounded shield around eachcable as described above, or by placing a single grounded shield aroundboth cables 30 a and 30 b.

Conventional capacitive sensing systems often drive the sensors using acurrent source and measure the resulting voltage across the sensorcapacitance. The invention, e.g. in the configurations shown in FIGS.4-6, uses a voltage source and a current measurement. The voltage sourcepreferably generates an AC triangular voltage waveform having a constantpeak amplitude, a constant frequency, and an alternating positive andnegative slope of constant slope, as shown in FIG. 7A, although otherwaveforms may also be used. An amplitude of 5V peak-to-peak andfrequency of 500 kHz are typical values. The voltage source preferablyhas a low output impedance to achieve a constant amplitude output undervarying load conditions, and can be implemented, e.g., using a highcurrent op-amp.

The shield driver may be implemented as an op-amp, preferably with lowoutput impedance. The shield driver may be integrated into the voltagesource for driving both the sensor wire and the shield conductor asdescribed above.

An example of a triangular voltage source waveform is shown in FIG. 7A,which ideally produces a square-wave current waveform as shown in FIG.7B, where the amplitude of the current waveform varies depending on themeasured capacitance. In practice the triangular voltage waveformresults in an imperfect current waveform more like the waveform shown inFIG. 7C. The current measurement circuit 21 may be configured to measurethe amplitude of the current waveform near the end of each half-cycle ata portion of the waveform where the amplitude has stabilized, to reducethe effect of such variable imperfections in the current waveform. Thecurrent measurement circuit 21 may be a current-to-voltage converter,preferably with low input impedance, high accuracy, and low distortion.

The capacitive sensor may be a conventional capacitive sensor or a thinfilm structure as described in U.S. patent application Ser. No.12/977,240, which is hereby incorporated by reference in its entirety.FIG. 8 illustrates connections of a triaxial cable 30 to a capacitivesensor, in this example a thin film sensor 40 comprising electrodes 41,42, 43 formed from thin film conductive layers with intervening thinfilm insulating layers 45. The sensor wire 31 is connected to thesensing electrode 41 of the sensor, the shield conductor 32 is connectedto a back guard electrode 42, and the grounded outer shield conductor isconnected to a shield electrode 43. A similar connection scheme may beused with a coaxial cable and with other types of sensors.

FIGS. 9A and 9B show an example embodiment of a sensor pair constructedas a single integrated unit, which may be used as a differential sensor.In these embodiments, the integrated unit includes two sensors 40 a and40 b, each having a separate sensing electrode 41 a, 41 b and a separateback guard electrode 42 a, 42 b. The two sensors of the sensor pairshare a single shield electrode 43 integrated with the sensor pair, oralternatively a conductive plate 46 on which the sensor pair is fixedcould serve as a shield electrode. The two sensors 40 a, 40 b arepreferably operated as a differential pair as described herein, whereeach sensor is driven by a voltage or current which is out-of-phase fromthe other sensor of the pair, preferably 180 degrees out-of-phase, and adifferential measurement is made to cancel out common mode errors.

FIG. 9B shows a top view of the sensor pair. The back guard and sensingelectrodes are formed in a rounded quadrilateral shape designed to fit,e.g. in corner positions around a final lens element of a lithographymachine. The electrodes may also be formed into circular shapes toproduce large area sensing electrodes.

Many alternatives to the above arrangements are possible. For example, acoaxial, triaxial, or cable with four or more conductors may be used. Acable with one or more shield conductors in a non-coaxial arrangementmay also be used, e.g. with the conductors arranged in a flatconfiguration with a central sensor wire with shield conductors oneither side. The shield driver may be separate from or integrated in thevoltage source. A single voltage source may be used to drive multiplesensors. This is particularly advantageous in the configurations withthe shield driver integrated with the voltage source, greatly reducingthe number of separate components used in the sensor system.

Some example calculations may be used to illustrate the improvement inthe performance of the invention. For a sensor with a 4 mm sensingsurface diameter at a nominal measuring distance of 0.1 mm results in anominal sensor capacitance of approximately 1 pF. A cable of type RG178and length five meters results in a cable capacitance between core andshield conductors of approximately 500 pF. A shield driver amplifierwith a gain bandwidth factor of 100 MHz and measurement frequency of 1MHz results in a gain of 0.99, i.e. with a deviation of 0.01 from unitygain. Using these example values, the steady-state performance of theconfigurations described above can be estimated. A conventional activeshielding configuration as shown in FIG. 3 results in a capacitancemeasurement: 1 pF+(1−0.99)×500 pF=6 pF. This large error is usuallycompensated for by calibration of the combined system of sensor andcable. The configuration with combined driver for both the sensor wireand shield conductor as shown in FIGS. 4-6 results in a capacitancemeasurement: 1 pF+(1−1)×500 pF=1 pF. In this example, measurement errorsof 500% are eliminated without requiring the combined sensor/cablesystem to be calibrated.

The performance of the configurations described above can also beestimated when an external disturbance causes a change in current in theshield conductor. For example, assuming a change in current in theshield conductor causes an additional 1% gain error in the shielddriver, the conventional active shielding configuration as shown in FIG.3 results in a capacitance measurement: 1 pF+(1−0.98)×500 pF=11 pF.Assuming the same 1% gain error in the shield/wire driver, theconfiguration with combined driver as shown in FIGS. 4-6 results in acapacitance measurement: 0.99×(1 pF+(1−1)×500 pF=0.99 pF. The representsan error deviation of 0.01 pF, which is only 1%. Sensitivity for cablelength/load is reduced to about 1%.

FIG. 10 shows a simplified diagram of capacitive sensors 1 and ameasurement circuit 103 implemented for distance measurement in acharged particle lithography machine. The lithography machine generatescharged particle beams for exposing a target 2 such as a silicon wafer,mounted on a stage 100 moveable in horizontal and vertical directions.The capacitive sensors are mounted onto a plate close to the finalelement of the projection lens 102 of the lithography machine, arrangedto measure the distance from the projection lens element to the surfaceof the wafer to be exposed. The sensors are connected via cable 30 tomeasurement system 103, which may include the voltage source 20 andcurrent measurement circuit 21 in any of the configurations describedherein. The measurement system 103 generates a current measurementsignal which is communicated to control system 104 which, on the basisof the measurement signal, controls the movement of the stage 100 tobring the target 2 to a desired distance from the projection lens of thelithography machine.

FIG. 11 shows a simplified block diagram illustrating the principalelements of a modular lithography apparatus 500. The lithographyapparatus 500 is preferably designed in a modular fashion to permit easeof maintenance. Major subsystems are preferably constructed inself-contained and removable modules, so that they can be removed fromthe lithography apparatus with as little disturbance to other subsystemsas possible. This is particularly advantageous for a lithography machineenclosed in a vacuum chamber, where access to the machine is limited.Thus, a faulty subsystem can be removed and replaced quickly, withoutunnecessarily disconnecting or disturbing other systems.

In the embodiment shown in FIG. 11, these modular subsystems include anillumination optics module 501 including a charged particle beam source301 and beam collimating system 302, an aperture array and condenserlens module 502 including aperture array 303 and condenser lens array304, a beam switching module 503 including a multi-aperture array 305and beamlet blanker array 306, and projection optics module 504including beam stop array 308, beam deflector array 309, and projectionlens arrays 310. The modules are designed to slide in and out from analignment frame. In the embodiment shown in FIG. 11, the alignment framecomprises an alignment inner subframe 505 suspended via vibrationdamping mounts 530 from an alignment outer subframe 506. A frame 508supports the alignment subframe 506 via vibration damping mounts 507.The target or wafer 330 rests on substrate support structure 509, whichis in turn placed on a chuck 510. The chuck 510 sits on the stage shortstroke 511 and long stroke 512 arranged to move the stage in varioushorizontal and vertical directions. The lithography machine is enclosedin vacuum chamber 335, which may include a mu metal shielding layer orlayers 515, and rests on base plate 520 supported by frame members 521.

In the embodiment shown in FIG. 11, five sets of capacitive sensors areused to measure position or distance of various elements in thelithography machine. Sensor set 401 is arranged to measure distancebetween the final lens element and the target 330, as shown e.g. in FIG.10. Sensor set 402 is arranged to measure distance between an opticalalignment sensor mounted near the final lens element and the target 330or chuck 510, to facilitate focusing of the alignment sensor beam foralignment of the target and stage. Sensor set 403 is arranged to measurethe position of the short stroke stage 511 in horizontal (X, Y-axis) andvertical (Z-axis) positions by measuring distances with respect to thelong stroke stage 512. Sensor set 404 is arranged for measuring positionof the suspended subframe 505 with respect to alignment subframe 506 inhorizontal and vertical positions by measurement with respect to thesubframe 505. Sensor set 405 is arranged for measuring position of theillumination optics module 501 in horizontal and vertical positions bymeasurement with respect to the subframe 505.

The capacitive sensors used in any of the applications shown in FIGS. 10and 11 are preferably thin film sensors, and may also be arranged inpairs for differential operation. The sensors may be of the type shownin FIG. 8, preferably connected to the cable 30 using the arrangementshown in FIG. 8. The sensors may also be constructed with multiplesensing electrodes on a signal substrate, such as the sensor pair shownin FIG. 9A, 9B. Use of a thin film construction enables the sensors tobe constructed at low cost, and enables the sensors to be placed innarrow spaces and on parts of the lithography machine not suitable forconventional sensors with larger dimensions. Operating the sensors in adifferential mode enables use of the sensors for measurement of distanceto an opposing surface that is not grounded, and does not require areturn electrical connection from the opposing surface to themeasurement system. The latter factor is advantageous in applicationswhere the sensor is arranged to measure distance to a movable part,where it is difficult or disadvantageous to make an electricalconnection to the moveable part for the sensing system.

These sets of sensors may be arranged in sets of six sensors to fromthree differential sensor pairs, for measurement in three axes, i.e.horizontal X, Y-axes and vertical Z-axis directions. This may beaccomplished by mounting the differential sensor pairs oriented formeasuring distance to each direction to a suitable opposing surface.Measurement signals from the sensors may be sued to adjust the positionof moveable parts of the lithography machine, e.g. using a piezomotor tomake small movements to obtain proper alignment of the part within thesystem.

Each set of sensors is connected via a cable 30 to a correspondingcurrent measurement circuit located in a cabinet outside the vacuumchamber and remote from the lithography machine. FIG. 19 shows anembodiment of a cabinet 600 housing circuit boards 601. Each circuitboard 601 provides the current measurement circuits for a capacitivesensor 40, a pair of circuit boards 602 providing the currentmeasurement circuits for a differential sensor pair. A signal generator605 provides the AC voltage signal for energizing the capacitivesensors, e.g. a triangular voltage waveform from voltage source 20 asdescribed herein. Each circuit board is connected via connector 612 andcable 30 to a thin film capacitive sensor 40. Current measurement outputsignals are output via another connector to analog-to-digital converter613 for conversion to digital signals for use in controlling thelithography machine. Power supply 610 provides power to the circuitboards via power supply connector 611.

The current measurement circuits 21, 21 a, 21 b may be implemented, forexample, as a current-to-voltage converter or a current-to-currentconverter. There are several factors which contribute to errors in suchmeasurement circuits. These include stray impedance in the inputcircuitry of the measuring circuit, a limited common mode rejectionratio (CMRR) of the input circuitry, and inaccuracy on the transferfunction of the measurement circuit independent of common mode.

FIG. 12 is a functional schematic diagram of a current measuring circuit70. The input current I_(CS) at input terminal 72 from an AC or DCcurrent source CS is to be measured by the circuit. A portion of thecurrent I_(CS) is diverted in the input circuitry of the measuringcircuit, this portion represented by current I_(CM). Voltagedisturbances on the input terminal 72 relative to the power supplyvoltages of the circuit cause variation in current I_(CM) flowingthrough the internal impedance Z_(CM). As a result, the current I_(meas)actually measured by the circuit is slightly less than the input currentI_(CS) desired to be measured, leading to a small error in themeasurement. The current I_(CM) results from stray impedances in theinput circuitry and common mode disturbances in the input signal. Asteady-state error can be corrected, but it is very difficult tocompensate for the current flow I_(CM) because the value of the strayimpedance is variable, depending on factors such as temperature, andcommon mode disturbances on the input also vary with time.

These measurement errors can be reduced by driving the supply voltageswith the same voltage present at the input terminal of the measurementcircuit. In this way, the disturbances on the input are transferred tothe supply voltages to reduce or eliminate currents flowing in themeasuring circuit caused by varying voltage differences between theinput signal and internal circuits in the measuring circuit.

The voltage supply terminals 75 and 76 of the current measuring circuitare connected to a power supply comprising voltage sources 77 a, 77 b. Avoltage source VD is provided to feed voltage disturbances at the inputterminal into the power supply, to that voltage differences between theinput signal and the measuring circuit supply voltages remain constant.The voltage source VD is connected to the measuring circuit powersupply, so that the power supply voltages are also driven by anyvoltages present at the input terminal of the measuring circuit. Thevoltage source VD may be provided by suitable feedback or feed forwardin the circuit.

FIG. 13 is a functional schematic diagram showing the current measuringcircuit of FIG. 12 where the voltage source VD is used to energize aload 71. Driving the load 71 with the voltage source VD results incurrent I_(CS) which is the current measured at the input terminal 72 ofthe circuit. Thus, the disturbance voltage VD which is coupled to thepower supply of the current measuring circuit is the voltage whichdrives the load 71 to produce the current I_(CS) to be measured. Feedingthe disturbance voltage into the power supply of the current measuringcircuit results in removing the varying voltage differences within thecurrent measuring circuit caused by the disturbance voltage. Thisremoves a source of error in the current measurement.

In the embodiment shown in FIG. 13, the disturbance voltage VD is alsosubtracted from the output of the current measuring circuit 70 by adifference circuit 79. The output signal at the output terminal 74 ofthe current measuring circuit 70 will have the disturbance voltage VDsuperimposed on the signal resulting from measuring the input currentI_(CS). Subtracting the disturbance voltage VD thus may be used toisolate the portion of the output signal representing the input currentmeasurement.

The embodiment in FIG. 13 shows two power supply terminals powered bytwo voltage sources 77 a, 77 b, typically supplying a positive and anegative DC voltage to positive and negative power supply terminals. Asingle power supply terminal and/or a single power supply voltage sourcemay be used instead. In this embodiment, the disturbance voltage is fedvia capacitors 78 a, 78 b to the power supply terminals so that the ACcomponent of the disturbance voltage is fed to the power supplyterminals while the DC component of the power supply voltages areisolated from the input terminal 72 and the voltage source VD. Inductorsmay also be used to isolate the AC component of the disturbance voltagefrom the power supply voltages, such as inductors 95, 96 shown in theembodiment in FIG. 15.

FIG. 14 is a diagram showing an embodiment of the current measuringcircuit implemented with an operational amplifier 80 (referred to as anopamp). A current source CS is connected to a negative terminal 82 ofthe opamp 80, and a positive input terminal 83 of the opamp is connectedto common. The opamp 80 has two power terminals 85 and 86, through whichthe operational amplifier 80 can be energized by the two voltage sources91 and 92.

The current source CS produces a current Ics to be measured. Animpedance 87 connected between the input terminal 82 and the outputterminal 84 provides negative feedback, and the opamp 80 operates tomaintain the voltage difference between the two input terminals 82 and83 at nearly zero. The opamp 80 has a very high input impedance so thatvery little of the current Ics flows into the opamp, but instead flowsthrough impedance 87. However, due to stray impedances in the inputcircuitry of the opamp 80 and a limited CMRR of the opamp, the opamp 80cannot completely eliminate the influences of common mode voltages onthe inputs.

In the embodiment shown, an AC voltage supply VG is used to drive theinput terminal 83. Because the opamp 80 is configured to maintain thetwo input terminals 82 and 83 at almost the same voltage, the voltage VDeffectively represents a common mode disturbance on the input terminals.The output of the voltage source VD, connected to the input terminal 83,is also connected to the opamp power supply circuit to feedforward thecommon mode disturbance voltages into the power supply voltages of theopamp 80. In this embodiment, the output of the voltage source VD isconnected via capacitors 93, 94 to couple the voltage at input terminal83 to the voltage supply to power supply terminals 85, 86. In this way,DC voltage sources 91, 92 supply a DC voltage to power supply terminals85, 86 while AC voltages present at input terminal 83 are also suppliedto the power supply terminals 85, 86. Inductors 95, 96 may also beincluded in the power supply as shown in the embodiment in FIG. 15 toprovide some isolation between the AC components of the feedforwardinput terminal voltage and the DC voltage sources 77a, 77b; 91, 92.

FIG. 15 shows an example of the embodiment of FIG. 14 used for measuringcurrent in a capacitive sensor system, such as shown in any of FIGS.3-6. The current to be measured is transferred to the current measuringcircuit 21 via cable 30, as the current measuring circuit 21 istypically located remotely from the capacitive sensor. The capacitivesensor may be a thin film capacitive sensor such as shown in FIGS. 8 and9. The cable 30 comprises a sensor wire 31 and a shield conductor 32 andhas a remote end and a local end. The sensor wire 31 is electricallyconnected to the capacitive sensor electrode 41 at the local end of thecable 30, and the shield conductor 32 is electrically connected to thecapacitive sensor guard electrode 42 at the local end of the cable 30.

The voltage source 20 energizes the shield conductor 32 at the remoteend of the cable 30 to energize the guard electrode 42. The voltagesource 20 also energizes the sensor wire 31 via the opamp 80 to energizethe sensing electrode 41 of the capacitive sensor. Because the opampmaintains the voltages at its input terminals 82, 83 at essentially thesame voltage, the sensor wire 31 and shield conductor 32 are alsoenergized at essentially the same voltage, virtually eliminatingcapacitive leakage current between them.

The output terminal of the voltage source 20 is connected to inputterminal 83, the shield conductor 32, and is also connected to the powersupply for the opamp 80 as described earlier, and is connected to adifference circuit 88 to subtract the signal from the voltage source 20from the output signal of the opamp 80.

The voltage source preferably provides a triangular voltage signal todrive the capacitive sensor, as described earlier. This results(ideally) in a square-wave current signal shown in FIG. 16A, asdescribed earlier. The triangular voltage output by the voltage source20 is present on input terminals 83 and 82 of the opamp 80, as shown inFIG. 16B. The output voltage Vout at the output terminal 84 of the opamp80 will include the triangular voltage present at the input terminalswith a square-wave superimposed due to the square-wave current flowingthrough the feedback impedance 87, as shown in FIG. 16C. In order toobtain a measurement signal with the same square-wave waveform as thecurrent generated at the capacitive sensor, the triangular voltagewaveform from the voltage source 20 is subtracted by the differencecircuit 88 from the signal at the output terminal 84.

FIG. 17 shows an embodiment of a voltage source and current measurementcircuit for a differential pair sensing system such as described herein.This circuit could also be used for single sensors not operating as adifferential pair. The circuitry is divided into an analog signalprocessing portion 50, and a digital signal processing portion 63 whichmay be implemented e.g. in a field programmable gate array.

A frequency reference F_(SYNC) is generated (e.g. at 2 MHz) and dividedin divider circuit 51 to generate multiple separate square-wave signalsat a lower frequency with certain predetermined phase offsets. In thisembodiment, four separate 500 kHz square-wave signals are generated with90 degree phase offsets. FIG. 18A shows an example of square wavefrequency reference signal, and FIGS. 18B-18E show example waveforms oflower frequency signals with 0, 90, 180 and 270 degree phase shifts.

Integrator circuit 52 generates a triangular voltage waveform from oneof the square-wave signals, and from this the amplifier circuits 53 aand 53 b generate two triangular voltage waveforms 180 degreesout-of-phase. For example, these two out-of-phase triangular voltagewaveforms may correspond to the outputs of voltage sources (e.g. 20, 20a, 20 b, VD) shown in any of FIG. 3-6, 13, 14 or 16 for driving a singlecapacitive sensor or load or two sensors/loads operated in adifferential pair. FIGS. 18F and 18G show an example of triangularwaveform outputs from amplifier circuits 53 a and 53 b. The triangularvoltage signals may be connected to energize the shield conductors 32,32 a, 32 b, and also the sensor wires 31, 31 a, 31 b viacurrent-to-voltage converters 54 a and 54 b, for energizing thecapacitive sensors or loads 40, 40 a, 40 b, 71 as shown in FIG. 3-6, 13,14 or 16.

The current-to-voltage converters 54 a and 54 b generate voltage signalsat their outputs representing a measurement of the current signals attheir inputs (i.e. the output signals 22, 22 a, 22 b, 74, 84 of FIG.3-6, 13, 14 or 16). The triangular voltage waveforms from the amplifiercircuits 53 a, 53 b are subtracted from the measured current signals atthe outputs of the current-to-voltage converters 54 a, 54 b bydifference circuits 55 a, 55 b to remove the triangular voltage signalfrom the current-to-voltage converter output signals, to isolate themeasured input current signal. FIGS. 18H and 18I show examples of theresulting measured current signal waveforms at the output of thedifference circuits 55 a, 55 b.

Selectors 56 a, 56 b use one or more of the phase shifted referencesignals generated by divider circuit 51, e.g. 180 and 270 degree shiftedreference signals shown in FIGS. 18D and 18E, to sample a portion ofeach cycle of the measured current signals shown in FIGS. 18H and 18I.The second half of each cycle of the measured current signals is sampledto obtain the amplitude for the portion of the cycle when it isgenerally steady-state at the maximum amplitude value.

When the circuit is used with a sensor pair operated in differentialmode, the sampling may be performed to switch between the two measuredcurrent signals, to accumulate the positive amplitudes in one signal(FIG. 18J) and the negative amplitudes in the other signal (FIG. 18K).Low pass filters 57 a, 57 b filter the sampled measured current signalsto implement an equivalent of a capacitor charging circuit with slopedetermined by the amplitude of the sampled portion of the measuredcurrent signal waveform.

Example waveforms at the output of amplifiers 58 a, 58 b are shown inFIGS. 18J and 18K. The waveforms rise (or fall) during each sampledperiod, the end values determined by the amplitude of the measuredcurrent signals. The outputs from the amplifiers 58 a, 58 b aresubtracted from each other and the resulting signal is converted to adigital signal by analog-to-digital converter 59. The resulting digitalsignal is output to the digital signal processing circuitry 63 forfurther processing such as calibration adjustment and scaling to resultin a useable measurement indicative of the sensor capacitance. Adder 61and window comparator 62 generate an error signal for use in the digitalsignal processing circuitry 63 for situations such as a cable short oropen fault.

The invention has been described by reference to certain embodimentsdiscussed above. It should be noted various constructions andalternatives have been described, which may be used with any of theembodiments described herein, as would be known to those of skill in theart. In particular, the current measuring circuits described in relationto FIGS. 12-14 may be used in any application requiring precisemeasurement of current, and the signal processing circuits described inrelation to FIG. 17 may be used for any application requiring isolationof an amplitude signal in an alternating signal. Furthermore, it will berecognized that these embodiments are susceptible to variousmodifications and alternative forms well known to those of skill in theart without departing from the spirit and scope of the invention.Accordingly, although specific embodiments have been described, theseare examples only and are not limiting upon the scope of the invention,which is defined in the accompanying claims.

The invention claimed is:
 1. A measurement system for measuring an inputelectrical current from a current source and generating a currentmeasurement signal, comprising a current measuring circuit having afirst input terminal connected to the current source for receiving theinput electrical current from the current source and an output terminalfor providing the current measurement signal, wherein the currentmeasuring circuit further comprises one or more power supply terminalsarranged to receive one or more voltages from a power supply forpowering the current measuring circuit, and wherein the currentmeasuring circuit further comprises a first voltage source coupled tothe one or more power supply terminals, the first voltage sourceproviding a disturbance voltage to the one or more power supplyterminals, the disturbance voltage representing a voltage at the firstinput terminal, wherein the first input terminal, the output terminaland the one or more power supply terminals are distinct, wherein thecurrent source, the power supply and the first voltage source aredistinct, and wherein the measurement circuit is configured to drive theone or more power supply voltages with a voltage essentially the same asa voltage present at the first input terminal by feeding the disturbancevoltage to the one or more power supply terminals.
 2. The measurementsystem of claim 1, further comprising a difference circuit arranged tosubtract a voltage generated by the first voltage source from a signalat the output terminal of the current measuring circuit to generate thecurrent measurement signal.
 3. The measurement system of claim 1,wherein the first voltage source is connected to the first inputterminal of the current measuring circuit for driving a load to form thecurrent source.
 4. The measurement system of claim 3, wherein the loadcomprises a capacitive sensor for generating a current which varies independence on distance between the capacitive sensor and a target. 5.The measurement system of claim 3, wherein the load is connected to thefirst input terminal of the current measuring circuit by a cablecomprising a sensor wire and a shield conductor, wherein the sensor wireis connected in series between the load and the first input terminal andthe shield conductor is connected to the first voltage source.
 6. Themeasurement system of claim 1, wherein an output terminal of the firstvoltage source is coupled via one or more capacitors to the one or morepower supply terminals of the current measuring circuit.
 7. Themeasurement system of claim 1, wherein the current measuring circuitcomprises a current-to-voltage converter.
 8. The measurement system ofclaim 1, wherein the current measuring circuit comprises an operationalamplifier, a negative input terminal of the operational amplifierserving as the first input terminal of the current measuring circuit andan output terminal of the operational amplifier serving as the outputterminal of the current measuring circuit, the operational amplifierfurther comprising a positive input terminal and one or more powersupply terminals, wherein the positive input terminal of the operationalamplifier is electrically connected to the one or more power supplyterminals of the operational amplifier.
 9. The measurement system ofclaim 8, wherein the positive input terminal of the operationalamplifier is electrically connected to the one or more power supplyterminals of the operational amplifier via one or more capacitors. 10.The measurement system of claim 1, wherein the first voltage sourcegenerates a voltage with a triangular waveform.
 11. The measurementsystem of claim 1, wherein the current source generates a current with asubstantially square waveform.
 12. A method for measuring an inputelectrical current from a current source and generating a currentmeasurement signal, comprising: providing the input electrical currentfrom the current source to a first input terminal of a current measuringcircuit, the current measuring circuit having one or more power supplyterminals arranged to receive one or more power supply voltages from apower supply for powering the current measuring circuit, and providing adisturbance voltage from a voltage source to the one or more powersupply terminals, the disturbance voltage representing a voltage at thefirst input terminal, hence driving the one or more power supplyvoltages with a voltage essentially the same as a voltage present at thefirst input terminal, and generating an output signal at an outputterminal of the current measuring circuit representing the inputelectrical current at the first input terminal of the current measuringcircuit, wherein the first input terminal, the output terminal and theone or more power supply terminals are distinct, and wherein the currentsource, the power supply and the first voltage source are distinct. 13.The method of claim 12, further comprising subtracting the disturbancevoltage from the output signal at the output terminal of the currentmeasuring circuit to generate the current measurement signal.
 14. Themethod of claim 12, further comprising driving a load with a voltage togenerate the input electrical current at the first input terminal of thecurrent measuring circuit.
 15. The method of claim 14, wherein the loadcomprises a capacitive sensor for generating a current which varies independence on distance between the capacitive sensor and a target. 16.The method of claim 14, further comprising connecting the load to thefirst input terminal of the current measuring circuit by a cablecomprising a sensor wire and a shield conductor, wherein the sensor wireis connected in series between the load and the first input terminal andthe shield conductor is energized with substantially the same voltageused to drive the load.
 17. The method of claim 12, wherein thedisturbance voltage is provided to the one or more power supplyterminals via one or more capacitors.
 18. The method of claim 12,wherein the disturbance voltage is isolated from the power supplyvoltages by one or more inductors.
 19. The measurement system of claim1, wherein the current measurement circuit is coupled to a cablecomprising a sensor wire and a shield conductor, wherein the sensor wireis coupled to the first input terminal and the shield conductor iscoupled to the first voltage source.
 20. The measurement system of claim19, wherein the cable is connected to a capacitive sensor at an end ofthe cable remote from the measurement circuit, the capacitive sensorcomprising a sensor electrode and a guard electrode, wherein the sensorelectrode is electrically connected to the sensor wire and the guardelectrode is electrically connected to the shield conductor.
 21. Themeasurement system of claim 1, wherein the power supply for powering thecurrent measurement circuit is arranged to supply a DC voltage to theone or more power supply terminals.
 22. The measurement system of claim1, wherein the disturbance voltage is an AC voltage.
 23. The measurementsystem of claim 1, wherein one or more inductors are provided betweenthe power supply and the first voltage source for isolating thedisturbance voltage from the one or more voltages from the power supply.24. The measurement system of claim 1, wherein the first voltage sourceis further coupled to a second input terminal of the current measurementcircuit.
 25. The measurement system of claim 1, wherein the currentmeasurement system comprises an operational amplifier and wherein thefirst input terminal, the output terminal and the one or more powersupply terminals are terminals of the operational amplifier.